1. Field of the Invention
This invention relates to an intake manifold leakage detection system of an internal combustion engine, more particularly to an intake manifold leakage detection system of an internal combustion engine for an outboard motor.
2. Description of the Related Art
In an internal combustion engine, intake air sucked in an intake pipe flows into an intake manifold, injected with fuel at an appropriate location and resulting air-fuel mixture flows into a cylinder combustion chamber where it is ignited and burns to drive the piston. When the intake manifold leaks, air enters from the leakage and hence the combustion state becomes different from that desired.
In view of the above, Japanese Laid-Open Patent Application No. 2000-104621 teaches detecting leakage of intake manifold by detecting in-cylinder pressure of respective cylinders. Specifically, in this prior art technique, an average of the detected in-cylinder pressures is calculated, each difference between the average and detected pressures is then calculated and is compared with a threshold value, and when any cylinder""s difference is found to exceed the threshold value, it is determined that a manifold portion connecting to that cylinder leaks.
However, this prior art requires pressure sensors installed in respective cylinders for detecting the in-cylinder pressure and hence, the configuration is disadvantageously complicated.
An object of the present invention is therefore to provide an intake manifold leakage detection system of an internal combustion engine which can accurately detect leakage of the intake manifold in a simple configuration.
For realizing this object, there is provided a system for detecting leakage of an intake manifold of an internal combustion engine having a secondary air passage, bypassing a throttle valve, at an air intake pipe connecting to the intake manifold and a secondary air control valve which regulates opening of the secondary air passage, comprising: engine starting determining means for determining whether the engine starts rotation; pressure detecting means for detecting intake manifold pressure and atmospheric pressure when the engine is determined not to start rotation; first change determining means for detecting the intake manifold pressure when the throttle valve is closed after the engine is determined to start rotation and for determining whether a change from the intake manifold pressure detected before the engine is determined to start rotation, is equal to or smaller than a predetermined first value; second change determining means for detecting the atmospheric pressure when the throttle valve is closed after the engine is determined to start rotation and for determining whether a change from the atmospheric pressure detected before the engine is determined to start rotation is equal to or smaller than a predetermined second value; manifold leakage possibility determining means for determining that the intake manifold may possibly leak when the change from the intake manifold pressure detected before the engine is determined to start rotation is equal to or smaller than the predetermined first value and the change from the atmospheric pressure detected before the engine is determined to start rotation is equal to or smaller than the predetermined second value; third change determining means for changing the opening of the secondary air passage by the secondary air control valve when it is determined that the intake manifold may possible leak and for determining whether a change of speed of the engine before and after the opening is changed, is equal to or smaller than a predetermined third value and a change of the intake manifold pressure before and after the opening is changed, is equal to or smaller than a predetermined fourth value; and manifold leaking determining means for determining that the intake manifold leaks when the change of speed of the engine is equal to or smaller than the predetermined third value and the change of the intake manifold pressure is equal to or smaller than the predetermined fourth value.